1. Field of the Invention
The present invention relates to transaxles, each of which includes an axle and a motor for driving the axle and has a function to change rotary speed or torque of the axle. A wheel hub drive unit to be attached to a hub of a single wheel serves as one of the transaxles. The wheel hub drive unit is integrally provided with a single axle to be drivingly connected to the wheel, and with a motor for driving the axle. The wheel hub drive unit is further integrally provided with a mechanical speed changing transmission, such as a gear transmission, interposed between the motor and the axle so as to change the rotary speed or torque of the axle. Additionally or alternatively, the wheel hub drive unit is provided with a plurality of motor generators serving as the motor so that the motor generators output variable torque for changing the rotary speed or torque of the axle. Another transaxle is provided with a pair of axles to be connected to respective drive wheels of a vehicle, and is provided with a plurality of motor generators serving as the motor so that the motor generators output variable torque for changing the rotary speed or torque of the axles. These transaxles are adaptable to a vehicle, especially, a working vehicle such as a lawn mower.
2. Related Art
As disclosed in Japanese Patent No. 3584106 (hereinafter, “Reference 1”), Japanese Laid-Open Gazette No. 2002-147596 (hereinafter, “Reference 2”), and U.S. Publication No. 2007/0084664 (hereinafter, “Reference 3”), there exists a well-known and conventional wheel hub drive unit, which is one type of transaxle, attached onto a hub of each of the drive wheels of a vehicle. The wheel hub drive unit includes an axle serving as a center axis of the wheel, a motor for driving the axle, and a high/low speed changing transmission, such as a gear transmission, interposed between the motor and the axle.
Each of the wheel hub drive units disclosed in References 1 and 2 is entirely expanded radially with respect to the axle because a motor output shaft of the motor and the axle are not coaxial, but rather are parallel to each other. A high speed gear train and a low speed gear train are interposed between the motor output shaft and the axle so as to serve as the high/low speed changing transmission.
Conversely, in the wheel hub drive unit disclosed in Reference 3, a rotary axis of the motor and the axle (spindle) are coaxial to each other so as to minimize the wheel hub drive unit radially with respect to the axle. Therefore, a hub casing of the wheel hub drive unit, fixed to a vehicle body, is disposed in a rim of the drive wheel of the vehicle, while the hub casing incorporates the entire motor and all components for transmitting power from the motor to the axle. The motor is an electric motor whose stator is fixed to the hub casing. A rotor serving as an output rotary member of the motor is fixed onto the spindle so as to be disposed between the spindle and the hub casing therearound.
Further, the wheel hub drive unit of Reference 3 includes a planetary gear mechanism serving as the speed changing transmission interposed between the motor and the axle. The planetary gear mechanism has a sun gear fixed on the spindle, and has a planetary gear carrier engaging with the rotor so as to be rotatably integral with the rotor. The spindle has a center axial hole into which a shift drum is inserted and is driven by another motor so as to axially move in the hole. The planetary gear mechanism has a ring gear. A ring gear clutch member (dog ring) is fitted to the hub casing axially slidable and unrotatable relative to the hub casing so as to selectively engage or disengage with and from the ring gear, and interlocks with a first shift lever pressed against the shift drum. On the other hand, a carrier clutch member (dog ring) is fitted to the sun gear axially slidable and unrotatable relative to the sun gear so as to selectively engage or disengage with and from the planetary gear carrier, and interlocks with a second shift lever pressed against the shift drum. The axial movement of the shift drum changes rotational positions of the respective first and second shift levers, thereby controlling the engagement and disengagement of the ring gear with and from the hub casing and the engagement and disengagement of the planetary gear carrier with and from the sun gear.
When the shift drum is disposed at its high speed position, the ring gear disengages from the hub casing, and meanwhile, the planetary gear carrier engages with the sun gear, thereby locking the rotor to the axle (spindle). Therefore, the axle is rotated at the same speed as the rotor. When the shift drum is disposed at its low speed position, the ring gear engages with the hub casing, and meanwhile, the planetary gear carrier disengages from the sun gear, so that the sun gear rotates planetary gears against the ring gear so as to rotate the axle slower than the rotor. When the shift drum is disposed at its neutral position, the ring gear disengages from the hub casing, and the planetary gear carrier disengages from the sun gear, thereby making the axle rotatable freely from the rotor.
Another conventional wheel hub drive unit, as disclosed in Japanese Laid-Open Gazette No. 2006-194257 (hereinafter, “Reference 4”), includes a plurality of motors which are independently controlled to change their output torque so as to change rotary speed or torque of the axle. Each of the motors includes exclusive stator and rotor, while the motors share a common rotor shaft, serving as a center axis of a drive wheel, and are aligned opposite to each other in the axial direction of the rotor shaft. More specifically, in the wheel hub drive unit shown in Reference 4, a first motor includes a first rotor and a first stator, a second motor includes a second rotor and second stator, and the first and second motors share a common rotor shaft serving as a common rotary axis of the first and second rotors, and are aligned opposite to each other in the axial direction of the rotor shaft. The first and second rotors are disposed to face the respective first and second stators. When at least one of the first and second stators is supplied with electric power, the corresponding first or second rotor facing the stator supplied with electric power is rotated, so that the rotary force of the rotating rotor is transmitted to the common rotor shaft. In this way, while the first and second motors are individually supplied with electric power to their respective stators, torques outputted from the respective motors are totaled, and the total torque rotates the common rotor shaft for driving the axle.
Further, in the wheel hub drive unit shown in Reference 4, while the first rotor is fixed onto the rotor shaft, the second rotor is rotatable relative to the rotor shaft, and is axially slidable along the rotor shaft. The relative rotation of the first and second rotors is converted into the axial slide of the second rotor relative to the first rotor, thereby applying a brake for braking the rotor shaft. Further, the second rotor can be rotated opposite to the rotation direction of the first rotor so as to brake the rotor shaft. The first motor is a motor generator which can regenerate electric power from the first stator when its first rotor receives rotary force of the axle. The second rotor is biased axially toward the first rotor by a spring so as to be initially rotatable integrally with the first rotor. Thus, the first motor can function as a regenerator only when its first rotor is rotatably integral with the second rotor due to the biasing force of the spring.